1. Field of the Invention
This invention relates generally to a differential gear mechanism for transmitting rotary power from a power source to the wheels of a vehicle so that the wheels can rotate at mutually differential speeds and to the manufacture of the differential mechanism.
2. Description of the Prior Art
A differential mechanism transmits rotary power differentially to output shafts from a ring gear driven by an external power source, such as an internal combustion engine or electric motor. The differential ring gear, usually a hypoid bevel gear, is secured to the differential case, which is generally produced of cast nodular or ductile iron for high torque applications. The case is secured to the ring gear and defines a chamber containing bevel pinions driveably connected to the case by pinion shaft or spider, depending on the number of bevel pinions used, a right-side bevel gear and a left-side bevel gear in continuous meshing engagement with the bevel pinions. To support the bevel pinions, the case has equally spaced holes, equal in number to the number of bevel pinions used and positioned such that the pinion shaft or spider legs pass through the bevel pinions and through the holes in a proper spatial relationship, allowing proper meshing of the bevel pinion and bevel side gears, enabling torque transfer from the differential ring gear to the differential case to the pinion shaft to the bevel pinion gears, the bevel side gears, and right-side and left-side output shaft.
Each shaft is driveably connected to a wheel of the vehicle. These shafts pass coaxially through openings in the case called hubs, which are supported circumferentially by bearings pressed into the differential carrier, supporting the entire differential assembly. Conventionally, the wall of the case is thick in the area where the pinion shaft or spider passes through the aforementioned holes, providing contact area sufficient to transmit the torsional load from the ring gear to the differential pin. It is preferable that there be no yielding of the case wall or the pinion shaft or spider in this area.
The inside surface of the one-piece cast differential case must be machined to tight tolerances. To accomplish this, complicated tools are inserted axially through a hub and radially through the windows. When the case or tool is rotated, this tool must be able to compensate for the different rotational radii required at different locations in the case, and this tool, or another tool, must be able to generate pockets where the pinion gears seat and in which they rotate with a hemispherical or flat-bottomed shape to match the back of the type of pinion gears used. Depending on the tolerances of the casting, particularly concentricity about its rotational axis and uniformity in wall thickness, some cases may need to be machined on the outside after the axis of rotation has been uniquely defined by the inside machining to achieve rotational balance.
Windows are cast into the one-piece case, which allows the pinion and side gears to be inserted for assembly. This is a manual operation requiring considerable dexterity.
Variations in the types of differential mechanisms, which include open, limited slip, and positive locking, require variations in the components that comprise the differential assembly. These variations have heretofore required that the case take different forms in order to accommodate the various components for each of applications.
There is a need in the industry, therefore, to improve the strength and stiffness of a differential assembly, to increase its torque capacity, to minimize its package space and to reduce the number of components that are unique to a particular application of the differential assembly, while minimally increasing or reducing total cost, including material and manufacturing cost. It is also desirable that weight is minimized and that NVH (noise, vibration and harshness) level be minimized.